In　this　study,　the　relationship　between　melt　flow　and　hump　formation　was　directly　revealed　during　laser　deep　penetration　welding　of　transparent　material.　In　partial-penetration　welding,　the　melt　flowed　obliquely　upwards　along　the　bottom　of　the　keyhole　to　the　rear　surface　of　the　molten　pool.　And　its　flow　rate　increased　first　and　then　decreased　with　the　increase　of　the　welding　speed.　There　was　a　one-to-one　correspondence　between　the　humps　formed　on　the　weld　surface　and　the　fluctuation　of　the　inclination　angle　of　the　front　keyhole　wall　(FKW)　near　the　Brewster　angle.　The　maximum　eruption　rate　of　the　laser-induced　vapor　on　the　FKW　exceeded　30　m/s.　In　full-penetration　welding,　the　melt　flow　direction　changed　and　humping　was　suppressed.　The　laser-induced　vapor　on　the　FKW　surface　can　act　on　the　rear　keyhole　wall,　and　this　is　the　main　reason　for　melt　flow.　When　the　inclination　angle　of　the　FKW　is　close　to　the　Brewster　angle,　the　amount　of　laser-induced　evaporation　on　the　FKW　will　increase　to　form　a　high　speed　jet　of　vapor　which　will　drive　melt　flow　in　the　molten　pool　behind　the　keyhole,　and　this　is　the　main　reason　for　hump　formation　on　the　rear　surface　of　the　molten　pool.